I'm interested in building a circuit to switch between 2 sim cards, and found a design guide by GSM module manufacturer Telit. (New design guide link here, see pg. 16+)

In particular, "Figure 6" of this guide shows the use of a p-channel MOSFET to switch the SIM Vcc on and off, and there's a separate multiplexer for the data, clock, and reset lines.

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I was wondering what the alternative ways of doing this were, and what affect they might have.

For example, would it be possible to remove the multiplexer and simply control the SIMs using the MOSFET, or would it be possible to use a switch on only the data line, and share the power, clock and reset between the SIM cards?

I know the voltage on SIM cards can be either 1.8V or 3V, so maybe sharing the Vcc line would cause issues?


That is an excellent guide and seems to cover the relevant points well.
After having read the guide, why do you think that you can cut corners and use simply interconnected pins? It MAY in fact be possible in practice, but it is not obvious that it would be safe or reliable (even though it may be :-) ).

Joining the pins from two SIMS together and powering or enabling one but not the other will result in the unpowered SIM loading the lines to and from the powered SIM and may lead to phantom powering of the "unpowered" SIM.

Note the note on page 17 that says that fig 6 (and so probably fig 7) will not work with 1.8V supply and that the circuit of fig8 is required for use with 1.8V supply.

A multipole mechanical relay would meet the need.

A 4 pole double throw mechanical switch would meet the need if manbual switching was OK. With care less than 4 poles could be used (eg Vcc could be left live with proper attention to levels.)

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  • \$\begingroup\$ Thanks @Russell - It was more curiosity than anything else. It's been hard finding information on SIM cards, I'm thinking I might need to be more general - maybe read more about general data transmission issues, or maybe start playing around with smart cards. \$\endgroup\$ – PDug Oct 25 '12 at 10:53
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    \$\begingroup\$ Its all about EMI - you could just connect it all up and it will work.. but in certain cases you will get very bad EMI that could cause loss of data. That guide is very good and up to high standards. \$\endgroup\$ – Piotr Kula Oct 25 '12 at 11:17

As soon as you have different power voltages for the SIMs (or power at one SIM and no power at the other SIM) you can not share any data, clock or reset lines, because protection diodes inside the SIM will shorten data, clock or reset signals to the lower power voltage of one of the SIMs.

It might be possible to share power, clock and data lines but use separate reset lines to deactivate the particular SIM you don't want to use by keeping its reset low (active low).

It is possible that your "1.8 V card" not only supports supply voltage class C (1.8V) but also class B (3V). In that case it can be operated also at 3V.
You can find out by analyzing its ATR (answer to reset).

The details of power-up sequence and procedure of switching between supply voltages is described in specification ETSI TS 102 221 (can be downloaded from http://www.etsi.org/WebSite/Standards/Standard.aspx) section "6.2 Supply voltage switching".

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  • \$\begingroup\$ Thanks @Curd! Do you think sharing power lines would be problematic if you had mixed 1.8V and 3V cards? I'm guessing there's no way of knowing unless you had some kind of detailed schematic for the SIM Card itself with max Vcc ratings? \$\endgroup\$ – PDug Oct 25 '12 at 10:48
  • \$\begingroup\$ @PDug: you don't need any schematic; you just have to analyze the ATR of your card. See my extended answer for details. \$\endgroup\$ – Curd Oct 25 '12 at 21:31

If you try to control the shared bus with VCC FETs only and leave one of the power FETs open, it will not short the signals, but... Instead, toggling on any lines on this SIM (data or clock or reset going high) will cause the voltage build-up (pulses will be rectified via built-in ESD clamping diodes), and the SIM would become active (or intermittently active), which will cause bus contention during host transactions. It will be a real mess. The dual SIM circuit will NOT work without full multiplexer. You need to follow the design guide, it is there for a reason.

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